Introduction
This article provides comprehensive information about heat exchangers.
Continue reading to learn about:
- What heat exchangers are
- The thermodynamics of heat exchangers
- Heat exchanger flow configurations
- And more...
Chapter One – What Are Heat Exchangers?
Heat exchangers are devices that transfer heat between two fluids while keeping them separated. They use a thermally conductive barrier to prevent fluid mixing while enabling efficient heat transfer. The working fluid either absorbs or releases heat to the process fluid, resulting in heating or cooling. Advances in technology and materials have led to diverse heat exchanger designs for specialized applications.
Heat transfer in exchangers involves both fluid convection and wall conduction. Design starts with calculating the heat transfer coefficient (U-factor) using Newton's cooling law. Engineers also use the logarithmic mean temperature difference (LMTD) to evaluate the temperature driving force. Fluid phases (liquid-to-liquid or vapor-to-liquid) affect both design and performance.
The fluids may be separated by a highly conductive wall (steel or aluminum) or may come into direct contact.
Unlike fuel, electric, or nuclear systems, heat exchangers don't generate heat. For example, boilers require both heat source and receiver to be fluids—substances that flow under stress, including liquids, gases, and vapors.
Chapter Two - Thermodynamics of Heat Exchangers
All heat exchangers—shell and tube, plate, finned tube, and air-cooled types—operate on thermodynamic principles. These laws govern thermal energy transfer between fluids separated by a solid barrier. Three key elements interact: hot fluid, cold fluid, and the dividing wall. Energy flows from hot to cold fluid, enabling heating, cooling, condensation, evaporation, and thermal recovery. Understanding these fundamentals is essential for optimizing heat exchanger performance in HVAC and industrial applications:
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First Law of Thermodynamics: This conservation law states energy cannot be created or destroyed, only transferred or transformed. For heat exchangers, it establishes the heat balance equation:
(Heat In) + (Heat Generation) = (Heat Out) + (Heat Accumulation)
This balance ensures accurate sizing, selection, and troubleshooting across industries like chemical processing and power generation.
- Second Law of Thermodynamics: This law introduces entropy, dictating that heat naturally flows from hotter to colder fluids. It determines maximum heat recovery potential and sets efficiency limits. The cold fluid warms while the hot fluid cools during this transfer.
Heat Transfer Mechanism
Heat exchangers primarily use conduction and convection, with radiation in special cases. The temperature gradient drives transfer, quantified by LMTD and U-value for design calculations.
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Approach Temperature: This minimum temperature difference between exiting and incoming fluids affects surface area needs and efficiency. Proper selection balances cost and performance, especially in economizers and condensers.
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Conduction: Heat transfers through molecular collisions in solid walls (tubes or plates). Fourier's Law describes this:
Q = -k A
Material selection considers conductivity, corrosion resistance, and process compatibility.
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Convection: Fluid motion transfers heat via forced (pumps/fans) or natural (buoyancy) convection. Newton's Cooling Law states:
Q = h A ΔT
The coefficient h depends on flow regime, velocity, viscosity, and geometry.
Typical heat transfer sequence:
- Hot fluid to wall (convection)
- Through wall (conduction)
- Wall to cold fluid (convection)
Advanced designs enhance transfer via turbulence, material selection, and flow configurations.
Performance Optimization: Modern heat exchangers require energy efficiency and reliability. Engineers consider thermodynamics plus fouling, pressure drop, and process compatibility. Simulations and maintenance ensure longevity. Selection criteria include heat duty, pressure drop, fluid compatibility, space, maintenance, and compliance with standards like ASME or FDA.
